Over the past two decades or so, a great deal of effort has been devoted to the study of how best to obtain "cohesive rubber failure" in textile-reinforced rubber goods. "Textile" is a general term for fibers which are converted to yarns, cords and fabrics (i.e. woven, knitted and non-woven fabrics). By "cohesive rubber failure" (hereafter, simply "cohesive failure") we refer to failure of a sample of reinforced rubber due to tearing of rubber from rubber, rather than tearing of rubber from textile ("adhesive failure"). Thus, cohesive failure is predicated upon the rubber composition itself rather than the bond between textile and rubber. This criterion of failure is particularly sought after in the manufacture of conveyor belts, hose and tires, all of which are normally reinforced with textiles made of rayon, nylon, aramid or polyester fibers. Polyester and aramid fibers are frequently preferred because of their high strength and high modulus which are particularly advantageous in these goods, but it has been found far more difficult to achieve sufficient adhesive bond strengths between polyester and rubber, or aramid and rubber, than between nylon and rubber, so as to result in cohesive failure.
It is a particularly noteworthy fact that, for over more than twenty years, bonding fabric to rubber is still effected with an aqueous dispersion of a resorcinol formaldehyde latex (R/F/L), with a wide spectrum of choices of resin catalyst (usually caustic and ammonia), in situ and pre-formed resins, molar ratios of resorcinol/formaldehyde (R/F), resin/latex (R/L) solids ratio, and adhesion-promoting additives. The difficulty in bonding polyester fibers to rubber is generally attributed to the presence of only hydroxyl (OH) and carboxyl (COOH) groups at the ends of the polyester molecules, while in nylon (for example) there is a relatively high frequency of amide (CONH) groups along the macromolecular chain. Aramid fibers are a special case which are not as satisfactorily coated as nylon, having instead, the adhesive characteristics of polyester fibers. Therefore, this invention is directed to polyester and aramid fibers, and most particularly to adhesive-activated (AA) polyester fibers in cords, woven, knitted and non-woven fabrics used to reinforce conveyor belts, tires and hose.
Many adhesives and bonding systems ("dips") have been used for fabrics made from synthetic linear polyester yarns. Most are cost-ineffective and additionally suffer from various other disadvantages such as toxicity in the case of adhesives based on glycidyl ethers, or water-soluble phenolic condensates; and/or instability as in the case of polyisocyanates, which has resulted in the use of water-insoluble reversibly blocked polyisocyanates (RBP) which, generally being solid, tend to precipitate in the baths in which the fabric is dip-coated. An RBP is so termed because the reactive isocyanate (NCO) group is blocked against significant reaction at low temperature below about 300.degree. F., and then the isocyanate is regenerated when the temperature is raised, usually above 350.degree. F. but below about 500.degree. F. The temperature at which a RBP will dissociate depends mostly on the blocking moiety (or substituting group).
This invention relates specifically to adhesive coatings for fabric-reinforced rubber goods in which the coating comprises an aqueous R/F/L and an RBP, whether the fabric is coated in a single dip ("one-step") bath or a double dip("two-step") bath.
Treatments which utilize phenol-blocked methylene-bis-(4-phenylisocyanate), (MDI for brevity), and the like are disclosed in U.S. Pat. No. 3,307,966, and the use of phenol-aldehyde blocked polyisocyanates are disclosed in U.S. Pat. No. 3,226,276, inter alia. Still other RBPs used are dimerized toluene diisocyanates, and methyl-ethyl-ketoxime blocked polyisocyanates. Additional details on RBPs are disclosed in "Synthesis of Blocked MDI Adducts, their DSC Evaluation and Effect of Pigmentation" by Taki Anagnostou and Ernest Jaul, J. Coatings Tech., pg 35-45, Vol. 53, Bo. 673, February 1981, the disclosure of which is incorporated by reference thereto as if fully set forth herein.
There is also disclosed therein that the unblocking temperature of the RBPs may be lowered by addition of a reactive polyol in equimolar amount with the unblocked MDI. Particularly disclosed is a polyoxyethylene glycol (Carbowax 400) inter alia. Addition of such a large amount of a polyol increases the viscosity of the R/F/L greatly and deleteriously affects the adhesion of rubber to the treated fabric so that cohesive failure cannot be obtained.
In a typical commercial process, polyester fabric is dipped in a first RBP bath in which solid finely ground RBP is dispersed with the aid of a dispersing agent, excess RBP removed, the RBP-coated fabric dried at about 300.degree. F., then the dried fabric is heat-set at a temperature below about 500.degree. F. In the second bath, heat-set RBP-coated fabric is dipped in an R/F/L, excess R/F/L is removed, dried at about 300.degree. F., and heat-set at a temperature below about 500.degree. F. so as to give excellent adhesion of the R/F/L to the RBP-coated fabric.
Though fabric which has been properly coated with either a one-step or a two-step coating (RBP, then R/F/L) has excellent adhesion to rubber if just the right viscosity is maintained so that there is increased dip coverage of the fabric, the risk of "flaking" is high. By "flaking"we refer to flakes or chunks of resin which are formed during the drying and heat-setting of the R/F/L dip on the fabric. This flaking is attributed to blisters caused when moisture entrapped by the R/F/L film volatilizes and expands the film locally (see "Adhesive Design for Improved Adhesive Treatment of Polyester in Conveyor Belts" by Stanhope, H. W., published at International Society of Industrial Yarn Manufacturers, Savannah, Ga., 1977). Flaking severely decreases the adhesion of the rubber to the fabric, thus resulting in adhesive failure (that is, between the rubber/dip interface, and/or the dip/fabric interface), rather than cohesive failure.
It was generally accepted that the best insurance against dip flaking was the use of long fabric drying times of about 3.5 minutes at about 250.degree. F., so that the rate of evaporation from each dip application (whether one-step or two-step dips) could be carefully controlled. These conditions are cost-ineffective due to relatively slow fabric throughput processing times.
All the causes of flaking are far from clear, but solutions to the problem have generally been directed to the chemistry of the baths used, their particular physical properties such as viscosity, and the precise conditions for drying and heat-setting the fabric after it is dipped. It was never suspected, nor did the prior art suggest, that the physical size of primary particles of solid RBP, or how they were held in suspension, might affect the problem of flaking.
Most particularly, since it is much harder to wet and disperse submicron size particles compared to larger particles (e.g. larger than 2 microns), and it is expected that smaller particles would form a film which is more likely than larger particles to trap moisture-causing blisters, there was no indication that sub-micron size particles might alleviate, rather than aggravate, the problem of flaking.
Still further, the physical problems of grinding an RBP to a submicron size are such that highly specialized equipment is required to do so, and sub-micron size RBPs are not commercially available.
Treating fabric with an R/F/L is a peculiar problem. The prior art failed to recognize that flaking was closely tied to adhesive failure. In the art of tire construction, cords in the fabric are arranged coplanarly in only one direction, as for example in fabric used for reinforcing tires, the problem of flaking is not generally critical, though it can be. Therefore, the relevance of the primary particle size of the RBP which usually is as large as about 5 microns, or even larger, was not noteworthy. RBPs with a primary particle size as small as 2 microns may be obtained commercially, and are currently in use.
It is recognized that, in the prior art, to save on the costs of a two-step process, numerous one-step processes have been suggested, but few proved usable. A widely used one-step dip is disclosed in U.S. Pat. No. 3,660,202 in which a water-soluble phenol is combined with a R/F/L. However, when an available RBP is combined with a R/F/L and the fabric coated in such a one-step bath, the coated fabric shows adhesive failure. This is attributed to the effect, on a molecular scale, of the RBP which interferes with the adhesion of the R/F/L. By coating the RBP in a first step, and the R/F/L in a second step, the R/F/L effectively sheaths the RBP-coated fabric, thus negating the effect of the RBP at the rubber-R/F/L interface; which is an accepted reason for the effectiveness of the two-step process. We know of no one-step adhesive composition for coating a polyester fabric which composition combines a solid RBP and a R/F/L, and which fabric in a vulcanizate is characterized by cohesive failure.
To avoid the operating inconveniences of a two-step process, specifically such as solid RBP settling out in the first bath, contamination of the second bath, and the like, and in addition, to save on operating costs, it is more desirable to provide a one-step process for coating polyester and aramid cord with a combination of the RBP and the R/F/L in a single bath without deleteriously affecting the properties of the coated and heat-set cord, and without destroying the useful life of the bath.
Though this invention is applicable to both one-step and two-step coating of fabrics with an aqueous R/F/L dispersion used in combination with a solid finely divided RBP, it is more specifically directed to a one-step process.